Greece on Friday banned vessels from leaving its shores to join a planned flotilla aimed at breaking Israel's sea blockade of the Gaza Strip.

Organizers say they hope to sail to Gaza in coming days with at least nine boats loaded with humanitarian aid for the Palestinians. Israel vows to prevent the flotilla from reaching Gaza.

Most of the ships are docked in Greece, where some have encountered administrative problems. About 300 people plan to join the convoy, including journalists, politicians, writers and religious figures.

Media reports said one boat carrying dozens of American activists left Greece for Gaza on Friday but was intercepted by the Greek coast guard.

Organizers of the flotilla accused Israel this week of sabotaging two ships. The organizers said Thursday they had discovered damage to the engine of the Irish boat Saoirse in the Turkish port of Gocek, where it had been berthed for the past few weeks.

They called the damage an act of sabotage committed by professional divers and blamed it on Israel. An Israeli foreign ministry official denied the allegations.

Israeli Defense Minister Ehud Barak said Thursday if the ships insist on challenging the Gaza blockade, they will be intercepted by the Israeli Navy and the organizers will be responsible for any injuries or damage.

Israeli officials say they hope to avoid a repeat of its botched raid on a Gaza-bound flotilla last year, when nine pro-Palestinian activists were killed in a confrontation with Israeli commandos who boarded a Turkish ship.

Israel says the naval blockade is necessary to prevent weapons from reaching the Palestinian militant group Hamas that rules Gaza.

Israel, the United States and the United Nations have urged the flotilla to sail to Israeli or Egyptian ports and transfer its cargo legally to Gaza over land.